With practiced fingers, Gilberto rolled the cigarette and licked it shut. He lighted the cigarette, inhaled deeply. As the drug took effect, a faint smile came across his face.

"It feels good," said Gilberto, 32, his voice becoming thick and smooth. "I feel a little more energetic than before."

It was 3 in the afternoon Thursday, and Gilberto had just awoken on the frayed, filthy mattress in the bedroom of his near-barren brick house in a northern Medellin neighborhood.

Although he had not eaten for 24 hours, Gilberto said he was not hungry. His body, however, craved basuco - the powder that is both his addiction and his stock in trade.

Basuco is a powerful cocaine derivative. Inexpensive and highly addictive, it often is compared to crack.

According to public health officials, the abuse of basuco has increased dramatically in the Andean nations where virtually all of the world's cocaine is produced, creating a public health crisis where none existed a decade ago.

In the early 1980s, government leaders in Colombia, Peru and Bolivia frequently argued that drug abuse was primarily a problem of U.S. consumption. Basuco has changed all that.

Now, U.S. officials say privately, drug abuse in the Andean countries is one reason nations such as Colombia are embracing U.S. anti-drug programs that they once shunned.

According to those who use it, basuco produces intensely pleasurable highs - as well as attacks of extreme paranoia.

"I often get this feeling of persecution," Gilberto said as he smoked on the counter in his kitchen, surrounded by drug paraphernalia and the light from a bare bulb - the only light bulb in the house. His eyes darted at the front door of the house because he said he was concerned that the police might break it down at any minute.

Though cocaine use is illegal in Colombia and police occasionally raid drug dens, Gilberto admitted that they never had called on his house.

Basuco does more than twist the mind, according to Colombian public health officials. The drug can be highly toxic - it frequently contains large amounts of the chemicals used to process cocaine, such as kerosene, gasoline and sulfuric acid.

"The composition of basuco is variable and extremely unpredictable," said Yolanda Torres, a professor of epidemiology at the University of Antioquia in Medellin.

Basuco abuse in Colombia is particularly pronounced in Medellin, the home of the drug cartel that supplies most of the world's cocaine.

"In Medellin you can see clearly the consumption of basuco as an epidemic phenomenon," Torres said in a 1987 study, the only nationwide study done on drug abuse.

Reliable measures of the number of basuco smokers are few.

Torres' study concluded that there were about 100,000 basuco smokers in Colombia, a nation of 31 million people. Other studies said there were five times that many.

Other than basuco use, cocaine abuse is relatively rare, health officials said. Although cocaine is much cheaper in Colombia than in the United States, it is still too costly for anybody but the wealthy.

"Our biggest problem is with basuco, and fundamentally that is a problem with the young and the poor," said Luis Javier Garcia Isaza, the director of the Medellin Metropolitan Health Department.

At one of the health department's clinics near downtown Medellin, a group of about 15 young addicts gathered last week as part of a program to help them quit. All but one were men.

"Basuco is great for those who sell it," said Juan Guillermo Vanegas, 24. "But it's not for those of us who use it."

Vanegas said he had tried to quit three times before.

Health officials said the relapse rate for those who went through public detoxification programs was close to 100 percent - so high that the metropolitan health department is considering ending sponsorship of treatment programs.

"We're putting more emphasis on prevention because of the failure of treatment programs," said Marta Cecelia Londono, the department's chief of special programs.

Those who are addicted say smoking basuco is a fierce habit that is hard to shake.

"You're smoking one cigarette while you're rolling another," said Vanegas. "You can't control it."

Such obsessive consumption was plainly evident Thursday afternoon as Gilberto agreed to allow a reporter and a photographer into the house where he sells packets of basuco for 25 cents each or cigarettes already laced with basuco for slightly more.

The house was adapted for the basuco smoker. The windows all had bars - "Those are to guard against police attacks," said Gilberto. Stained blankets hung over every window to keep the interior dark and to prevent outsiders from looking in. Visitors announced themselves at the door with a whistle.

The only ornaments on the walls were a depiction of the Last Supper in the living room and the Virgin Mary in the bedroom. The only visible furnishings were a mattress, a side table and a crude wooden bench; the rest of the furniture was locked into a side room to prevent the basuco smokers from destroying it.

The house did not escape abuse: The toilet overflowed with feces, electrical outlets dangled by frayed wires and empty bottles of aguardiente - a stiff Colombian liquor - were scattered in the kitchen.

"One time I was confused and I thought my clothes were on fire, so I tore them off," said Gilberto, who alternately drummed his fingers on his knees and twisted a chain of keys around his hands. He wore a plaid shirt haphazardly buttoned, and he did not appear to have bathed recently.

Gilberto said that he wanted to quit smoking the drug, but that the desire was too strong. And so he kept selling basuco in order to supply his habit.

"It's like when your stomach is asking for food - you feed it," he said. "That's the way I feel about smoking."

A few other people stopped by to smoke. Gilberto did not keep his basuco packets in the house, so when new arrivals came, he would leave the house to obtain the packets.

He also employed runners, like Eduardo, to fetch the drugs.

Eduardo, who like Gilberto is in his early 30s, had sunken, unshaved cheeks and wore ragged clothing. Like many basuco addicts, he was emaciated because the drug took away his appetite and his money. He said little, but he smoked constantly.

One customer, Carlos, 24, appeared to be a little healthier than the hosts of the house - he said he had smoked basuco for 10 years, while Gilberto said his habit extended back more than 15 years. Carlos said he recently attended drug therapy at a public hospital, but was only partially successful.

Carlos, who said he sometimes worked as a mechanic, said that the public rehabilitation programs were inadequate, meeting only several times a week, and that he could not afford a private treatment program. The private clinics charge up to $100 a day for treatment. Colombia's minimum wage is $75 a month.

He rolled a cigarette that contained basuco mixed with marijuana.

"I feel like I'm compensating a need," he said.

He smoked the cigarette until it was no more than a scrap of paper that he pinched between his fingernails.